path: root/lib/sqlalchemy/util/langhelpers.py

# util/langhelpers.py
# Copyright (C) 2005-2017 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""Routines to help with the creation, loading and introspection of
modules, classes, hierarchies, attributes, functions, and methods.

"""
import itertools
import inspect
import operator
import re
import sys
import types
import warnings
from functools import update_wrapper
from .. import exc
import hashlib
from . import compat
from . import _collections


def md5_hex(x):
    if compat.py3k:
        x = x.encode('utf-8')
    m = hashlib.md5()
    m.update(x)
    return m.hexdigest()


class safe_reraise(object):
    """Reraise an exception after invoking some
    handler code.

    Stores the existing exception info before
    invoking so that it is maintained across a potential
    coroutine context switch.

    e.g.::

        try:
            sess.commit()
        except:
            with safe_reraise():
                sess.rollback()

    """

    __slots__ = ('warn_only', '_exc_info')

    def __init__(self, warn_only=False):
        self.warn_only = warn_only

    def __enter__(self):
        self._exc_info = sys.exc_info()

    def __exit__(self, type_, value, traceback):
        # see #2703 for notes
        if type_ is None:
            exc_type, exc_value, exc_tb = self._exc_info
            self._exc_info = None   # remove potential circular references
            if not self.warn_only:
                compat.reraise(exc_type, exc_value, exc_tb)
        else:
            if not compat.py3k and self._exc_info and self._exc_info[1]:
                # emulate Py3K's behavior of telling us when an exception
                # occurs in an exception handler.
                warn(
                    "An exception has occurred during handling of a "
                    "previous exception.  The previous exception "
                    "is:\n %s %s\n" % (self._exc_info[0], self._exc_info[1]))
            self._exc_info = None   # remove potential circular references
            compat.reraise(type_, value, traceback)


def decode_slice(slc):
    """decode a slice object as sent to __getitem__.

    takes into account the 2.5 __index__() method, basically.

    """
    ret = []
    for x in slc.start, slc.stop, slc.step:
        if hasattr(x, '__index__'):
            x = x.__index__()
        ret.append(x)
    return tuple(ret)


def _unique_symbols(used, *bases):
    used = set(used)
    for base in bases:
        pool = itertools.chain((base,),
                               compat.itertools_imap(lambda i: base + str(i),
                                                     range(1000)))
        for sym in pool:
            if sym not in used:
                used.add(sym)
                yield sym
                break
        else:
            raise NameError("exhausted namespace for symbol base %s" % base)


def map_bits(fn, n):
    """Call the given function given each nonzero bit from n."""

    while n:
        b = n & (~n + 1)
        yield fn(b)
        n ^= b


def decorator(target):
    """A signature-matching decorator factory."""

    def decorate(fn):
        if not inspect.isfunction(fn):
            raise Exception("not a decoratable function")
        spec = compat.inspect_getfullargspec(fn)
        names = tuple(spec[0]) + spec[1:3] + (fn.__name__,)
        targ_name, fn_name = _unique_symbols(names, 'target', 'fn')

        metadata = dict(target=targ_name, fn=fn_name)
        metadata.update(format_argspec_plus(spec, grouped=False))
        metadata['name'] = fn.__name__
        code = """\
def %(name)s(%(args)s):
    return %(target)s(%(fn)s, %(apply_kw)s)
""" % metadata
        decorated = _exec_code_in_env(code,
                                      {targ_name: target, fn_name: fn},
                                      fn.__name__)
        decorated.__defaults__ = getattr(fn, 'im_func', fn).__defaults__
        decorated.__wrapped__ = fn
        return update_wrapper(decorated, fn)
    return update_wrapper(decorate, target)


def _exec_code_in_env(code, env, fn_name):
    exec(code, env)
    return env[fn_name]


def public_factory(target, location):
    """Produce a wrapping function for the given cls or classmethod.

    Rationale here is so that the __init__ method of the
    class can serve as documentation for the function.

    """
    if isinstance(target, type):
        fn = target.__init__
        callable_ = target
        doc = "Construct a new :class:`.%s` object. \n\n"\
            "This constructor is mirrored as a public API function; "\
            "see :func:`~%s` "\
            "for a full usage and argument description." % (
                target.__name__, location, )
    else:
        fn = callable_ = target
        doc = "This function is mirrored; see :func:`~%s` "\
            "for a description of arguments." % location

    location_name = location.split(".")[-1]
    spec = compat.inspect_getfullargspec(fn)
    del spec[0][0]
    metadata = format_argspec_plus(spec, grouped=False)
    metadata['name'] = location_name
    code = """\
def %(name)s(%(args)s):
    return cls(%(apply_kw)s)
""" % metadata
    env = {'cls': callable_, 'symbol': symbol}
    exec(code, env)
    decorated = env[location_name]
    decorated.__doc__ = fn.__doc__
    decorated.__module__ = "sqlalchemy" + location.rsplit(".", 1)[0]
    if compat.py2k or hasattr(fn, '__func__'):
        fn.__func__.__doc__ = doc
    else:
        fn.__doc__ = doc
    return decorated


class PluginLoader(object):

    def __init__(self, group, auto_fn=None):
        self.group = group
        self.impls = {}
        self.auto_fn = auto_fn

    def load(self, name):
        if name in self.impls:
            return self.impls[name]()

        if self.auto_fn:
            loader = self.auto_fn(name)
            if loader:
                self.impls[name] = loader
                return loader()

        try:
            import pkg_resources
        except ImportError:
            pass
        else:
            for impl in pkg_resources.iter_entry_points(
                    self.group, name):
                self.impls[name] = impl.load
                return impl.load()

        raise exc.NoSuchModuleError(
            "Can't load plugin: %s:%s" %
            (self.group, name))

    def register(self, name, modulepath, objname):
        def load():
            mod = compat.import_(modulepath)
            for token in modulepath.split(".")[1:]:
                mod = getattr(mod, token)
            return getattr(mod, objname)
        self.impls[name] = load


def get_cls_kwargs(cls, _set=None):
    r"""Return the full set of inherited kwargs for the given `cls`.

    Probes a class's __init__ method, collecting all named arguments.  If the
    __init__ defines a \**kwargs catch-all, then the constructor is presumed
    to pass along unrecognized keywords to its base classes, and the
    collection process is repeated recursively on each of the bases.

    Uses a subset of inspect.getargspec() to cut down on method overhead.
    No anonymous tuple arguments please !

    """
    toplevel = _set is None
    if toplevel:
        _set = set()

    ctr = cls.__dict__.get('__init__', False)

    has_init = ctr and isinstance(ctr, types.FunctionType) and \
        isinstance(ctr.__code__, types.CodeType)

    if has_init:
        names, has_kw = inspect_func_args(ctr)
        _set.update(names)

        if not has_kw and not toplevel:
            return None

    if not has_init or has_kw:
        for c in cls.__bases__:
            if get_cls_kwargs(c, _set) is None:
                break

    _set.discard('self')
    return _set


try:
    # TODO: who doesn't have this constant?
    from inspect import CO_VARKEYWORDS

    def inspect_func_args(fn):
        co = fn.__code__
        nargs = co.co_argcount
        names = co.co_varnames
        args = list(names[:nargs])
        has_kw = bool(co.co_flags & CO_VARKEYWORDS)
        return args, has_kw

except ImportError:
    def inspect_func_args(fn):
        names, _, has_kw, _ = inspect.getargspec(fn)
        return names, bool(has_kw)


def get_func_kwargs(func):
    """Return the set of legal kwargs for the given `func`.

    Uses getargspec so is safe to call for methods, functions,
    etc.

    """

    return compat.inspect_getargspec(func)[0]


def get_callable_argspec(fn, no_self=False, _is_init=False):
    """Return the argument signature for any callable.

    All pure-Python callables are accepted, including
    functions, methods, classes, objects with __call__;
    builtins and other edge cases like functools.partial() objects
    raise a TypeError.

    """
    if inspect.isbuiltin(fn):
        raise TypeError("Can't inspect builtin: %s" % fn)
    elif inspect.isfunction(fn):
        if _is_init and no_self:
            spec = compat.inspect_getargspec(fn)
            return compat.ArgSpec(spec.args[1:], spec.varargs,
                                  spec.keywords, spec.defaults)
        else:
            return compat.inspect_getargspec(fn)
    elif inspect.ismethod(fn):
        if no_self and (_is_init or fn.__self__):
            spec = compat.inspect_getargspec(fn.__func__)
            return compat.ArgSpec(spec.args[1:], spec.varargs,
                                  spec.keywords, spec.defaults)
        else:
            return compat.inspect_getargspec(fn.__func__)
    elif inspect.isclass(fn):
        return get_callable_argspec(
            fn.__init__, no_self=no_self, _is_init=True)
    elif hasattr(fn, '__func__'):
        return compat.inspect_getargspec(fn.__func__)
    elif hasattr(fn, '__call__'):
        if inspect.ismethod(fn.__call__):
            return get_callable_argspec(fn.__call__, no_self=no_self)
        else:
            raise TypeError("Can't inspect callable: %s" % fn)
    else:
        raise TypeError("Can't inspect callable: %s" % fn)


def format_argspec_plus(fn, grouped=True):
    """Returns a dictionary of formatted, introspected function arguments.

    A enhanced variant of inspect.formatargspec to support code generation.

    fn
       An inspectable callable or tuple of inspect getargspec() results.
    grouped
      Defaults to True; include (parens, around, argument) lists

    Returns:

    args
      Full inspect.formatargspec for fn
    self_arg
      The name of the first positional argument, varargs[0], or None
      if the function defines no positional arguments.
    apply_pos
      args, re-written in calling rather than receiving syntax.  Arguments are
      passed positionally.
    apply_kw
      Like apply_pos, except keyword-ish args are passed as keywords.

    Example::

      >>> format_argspec_plus(lambda self, a, b, c=3, **d: 123)
      {'args': '(self, a, b, c=3, **d)',
       'self_arg': 'self',
       'apply_kw': '(self, a, b, c=c, **d)',
       'apply_pos': '(self, a, b, c, **d)'}

    """
    if compat.callable(fn):
        spec = compat.inspect_getfullargspec(fn)
    else:
        # we accept an existing argspec...
        spec = fn
    args = inspect.formatargspec(*spec)
    if spec[0]:
        self_arg = spec[0][0]
    elif spec[1]:
        self_arg = '%s[0]' % spec[1]
    else:
        self_arg = None

    if compat.py3k:
        apply_pos = inspect.formatargspec(spec[0], spec[1],
                                          spec[2], None, spec[4])
        num_defaults = 0
        if spec[3]:
            num_defaults += len(spec[3])
        if spec[4]:
            num_defaults += len(spec[4])
        name_args = spec[0] + spec[4]
    else:
        apply_pos = inspect.formatargspec(spec[0], spec[1], spec[2])
        num_defaults = 0
        if spec[3]:
            num_defaults += len(spec[3])
        name_args = spec[0]

    if num_defaults:
        defaulted_vals = name_args[0 - num_defaults:]
    else:
        defaulted_vals = ()

    apply_kw = inspect.formatargspec(name_args, spec[1], spec[2],
                                     defaulted_vals,
                                     formatvalue=lambda x: '=' + x)
    if grouped:
        return dict(args=args, self_arg=self_arg,
                    apply_pos=apply_pos, apply_kw=apply_kw)
    else:
        return dict(args=args[1:-1], self_arg=self_arg,
                    apply_pos=apply_pos[1:-1], apply_kw=apply_kw[1:-1])


def format_argspec_init(method, grouped=True):
    """format_argspec_plus with considerations for typical __init__ methods

    Wraps format_argspec_plus with error handling strategies for typical
    __init__ cases::

      object.__init__ -> (self)
      other unreflectable (usually C) -> (self, *args, **kwargs)

    """
    if method is object.__init__:
        args = grouped and '(self)' or 'self'
    else:
        try:
            return format_argspec_plus(method, grouped=grouped)
        except TypeError:
            args = (grouped and '(self, *args, **kwargs)'
                    or 'self, *args, **kwargs')
    return dict(self_arg='self', args=args, apply_pos=args, apply_kw=args)


def getargspec_init(method):
    """inspect.getargspec with considerations for typical __init__ methods

    Wraps inspect.getargspec with error handling for typical __init__ cases::

      object.__init__ -> (self)
      other unreflectable (usually C) -> (self, *args, **kwargs)

    """
    try:
        return compat.inspect_getargspec(method)
    except TypeError:
        if method is object.__init__:
            return (['self'], None, None, None)
        else:
            return (['self'], 'args', 'kwargs', None)


def unbound_method_to_callable(func_or_cls):
    """Adjust the incoming callable such that a 'self' argument is not
    required.

    """

    if isinstance(func_or_cls, types.MethodType) and not func_or_cls.__self__:
        return func_or_cls.__func__
    else:
        return func_or_cls


def generic_repr(obj, additional_kw=(), to_inspect=None, omit_kwarg=()):
    """Produce a __repr__() based on direct association of the __init__()
    specification vs. same-named attributes present.

    """
    if to_inspect is None:
        to_inspect = [obj]
    else:
        to_inspect = _collections.to_list(to_inspect)

    missing = object()

    pos_args = []
    kw_args = _collections.OrderedDict()
    vargs = None
    for i, insp in enumerate(to_inspect):
        try:
            (_args, _vargs, vkw, defaults) = \
                compat.inspect_getargspec(insp.__init__)
        except TypeError:
            continue
        else:
            default_len = defaults and len(defaults) or 0
            if i == 0:
                if _vargs:
                    vargs = _vargs
                if default_len:
                    pos_args.extend(_args[1:-default_len])
                else:
                    pos_args.extend(_args[1:])
            else:
                kw_args.update([
                    (arg, missing) for arg in _args[1:-default_len]
                ])

            if default_len:
                kw_args.update([
                    (arg, default)
                    for arg, default
                    in zip(_args[-default_len:], defaults)
                ])
    output = []

    output.extend(repr(getattr(obj, arg, None)) for arg in pos_args)

    if vargs is not None and hasattr(obj, vargs):
        output.extend([repr(val) for val in getattr(obj, vargs)])

    for arg, defval in kw_args.items():
        if arg in omit_kwarg:
            continue
        try:
            val = getattr(obj, arg, missing)
            if val is not missing and val != defval:
                output.append('%s=%r' % (arg, val))
        except Exception:
            pass

    if additional_kw:
        for arg, defval in additional_kw:
            try:
                val = getattr(obj, arg, missing)
                if val is not missing and val != defval:
                    output.append('%s=%r' % (arg, val))
            except Exception:
                pass

    return "%s(%s)" % (obj.__class__.__name__, ", ".join(output))


class portable_instancemethod(object):
    """Turn an instancemethod into a (parent, name) pair
    to produce a serializable callable.

    """

    __slots__ = 'target', 'name', 'kwargs', '__weakref__'

    def __getstate__(self):
        return {'target': self.target, 'name': self.name,
                'kwargs': self.kwargs}

    def __setstate__(self, state):
        self.target = state['target']
        self.name = state['name']
        self.kwargs = state.get('kwargs', ())

    def __init__(self, meth, kwargs=()):
        self.target = meth.__self__
        self.name = meth.__name__
        self.kwargs = kwargs

    def __call__(self, *arg, **kw):
        kw.update(self.kwargs)
        return getattr(self.target, self.name)(*arg, **kw)


def class_hierarchy(cls):
    """Return an unordered sequence of all classes related to cls.

    Traverses diamond hierarchies.

    Fibs slightly: subclasses of builtin types are not returned.  Thus
    class_hierarchy(class A(object)) returns (A, object), not A plus every
    class systemwide that derives from object.

    Old-style classes are discarded and hierarchies rooted on them
    will not be descended.

    """
    if compat.py2k:
        if isinstance(cls, types.ClassType):
            return list()

    hier = {cls}
    process = list(cls.__mro__)
    while process:
        c = process.pop()
        if compat.py2k:
            if isinstance(c, types.ClassType):
                continue
            bases = (_ for _ in c.__bases__
                     if _ not in hier and not isinstance(_, types.ClassType))
        else:
            bases = (_ for _ in c.__bases__ if _ not in hier)

        for b in bases:
            process.append(b)
            hier.add(b)

        if compat.py3k:
            if c.__module__ == 'builtins' or not hasattr(c, '__subclasses__'):
                continue
        else:
            if c.__module__ == '__builtin__' or not hasattr(
                    c, '__subclasses__'):
                continue

        for s in [_ for _ in c.__subclasses__() if _ not in hier]:
            process.append(s)
            hier.add(s)
    return list(hier)


def iterate_attributes(cls):
    """iterate all the keys and attributes associated
       with a class, without using getattr().

       Does not use getattr() so that class-sensitive
       descriptors (i.e. property.__get__()) are not called.

    """
    keys = dir(cls)
    for key in keys:
        for c in cls.__mro__:
            if key in c.__dict__:
                yield (key, c.__dict__[key])
                break


def monkeypatch_proxied_specials(into_cls, from_cls, skip=None, only=None,
                                 name='self.proxy', from_instance=None):
    """Automates delegation of __specials__ for a proxying type."""

    if only:
        dunders = only
    else:
        if skip is None:
            skip = ('__slots__', '__del__', '__getattribute__',
                    '__metaclass__', '__getstate__', '__setstate__')
        dunders = [m for m in dir(from_cls)
                   if (m.startswith('__') and m.endswith('__') and
                       not hasattr(into_cls, m) and m not in skip)]

    for method in dunders:
        try:
            fn = getattr(from_cls, method)
            if not hasattr(fn, '__call__'):
                continue
            fn = getattr(fn, 'im_func', fn)
        except AttributeError:
            continue
        try:
            spec = compat.inspect_getargspec(fn)
            fn_args = inspect.formatargspec(spec[0])
            d_args = inspect.formatargspec(spec[0][1:])
        except TypeError:
            fn_args = '(self, *args, **kw)'
            d_args = '(*args, **kw)'

        py = ("def %(method)s%(fn_args)s: "
              "return %(name)s.%(method)s%(d_args)s" % locals())

        env = from_instance is not None and {name: from_instance} or {}
        compat.exec_(py, env)
        try:
            env[method].__defaults__ = fn.__defaults__
        except AttributeError:
            pass
        setattr(into_cls, method, env[method])


def methods_equivalent(meth1, meth2):
    """Return True if the two methods are the same implementation."""

    return getattr(meth1, '__func__', meth1) is getattr(
        meth2, '__func__', meth2)


def as_interface(obj, cls=None, methods=None, required=None):
    """Ensure basic interface compliance for an instance or dict of callables.

    Checks that ``obj`` implements public methods of ``cls`` or has members
    listed in ``methods``. If ``required`` is not supplied, implementing at
    least one interface method is sufficient. Methods present on ``obj`` that
    are not in the interface are ignored.

    If ``obj`` is a dict and ``dict`` does not meet the interface
    requirements, the keys of the dictionary are inspected. Keys present in
    ``obj`` that are not in the interface will raise TypeErrors.

    Raises TypeError if ``obj`` does not meet the interface criteria.

    In all passing cases, an object with callable members is returned.  In the
    simple case, ``obj`` is returned as-is; if dict processing kicks in then
    an anonymous class is returned.

    obj
      A type, instance, or dictionary of callables.
    cls
      Optional, a type.  All public methods of cls are considered the
      interface.  An ``obj`` instance of cls will always pass, ignoring
      ``required``..
    methods
      Optional, a sequence of method names to consider as the interface.
    required
      Optional, a sequence of mandatory implementations. If omitted, an
      ``obj`` that provides at least one interface method is considered
      sufficient.  As a convenience, required may be a type, in which case
      all public methods of the type are required.

    """
    if not cls and not methods:
        raise TypeError('a class or collection of method names are required')

    if isinstance(cls, type) and isinstance(obj, cls):
        return obj

    interface = set(methods or [m for m in dir(cls) if not m.startswith('_')])
    implemented = set(dir(obj))

    complies = operator.ge
    if isinstance(required, type):
        required = interface
    elif not required:
        required = set()
        complies = operator.gt
    else:
        required = set(required)

    if complies(implemented.intersection(interface), required):
        return obj

    # No dict duck typing here.
    if not isinstance(obj, dict):
        qualifier = complies is operator.gt and 'any of' or 'all of'
        raise TypeError("%r does not implement %s: %s" % (
            obj, qualifier, ', '.join(interface)))

    class AnonymousInterface(object):
        """A callable-holding shell."""

    if cls:
        AnonymousInterface.__name__ = 'Anonymous' + cls.__name__
    found = set()

    for method, impl in dictlike_iteritems(obj):
        if method not in interface:
            raise TypeError("%r: unknown in this interface" % method)
        if not compat.callable(impl):
            raise TypeError("%r=%r is not callable" % (method, impl))
        setattr(AnonymousInterface, method, staticmethod(impl))
        found.add(method)

    if complies(found, required):
        return AnonymousInterface

    raise TypeError("dictionary does not contain required keys %s" %
                    ', '.join(required - found))


class memoized_property(object):
    """A read-only @property that is only evaluated once."""

    def __init__(self, fget, doc=None):
        self.fget = fget
        self.__doc__ = doc or fget.__doc__
        self.__name__ = fget.__name__

    def __get__(self, obj, cls):
        if obj is None:
            return self
        obj.__dict__[self.__name__] = result = self.fget(obj)
        return result

    def _reset(self, obj):
        memoized_property.reset(obj, self.__name__)

    @classmethod
    def reset(cls, obj, name):
        obj.__dict__.pop(name, None)


def memoized_instancemethod(fn):
    """Decorate a method memoize its return value.

    Best applied to no-arg methods: memoization is not sensitive to
    argument values, and will always return the same value even when
    called with different arguments.

    """

    def oneshot(self, *args, **kw):
        result = fn(self, *args, **kw)
        memo = lambda *a, **kw: result
        memo.__name__ = fn.__name__
        memo.__doc__ = fn.__doc__
        self.__dict__[fn.__name__] = memo
        return result
    return update_wrapper(oneshot, fn)


class group_expirable_memoized_property(object):
    """A family of @memoized_properties that can be expired in tandem."""

    def __init__(self, attributes=()):
        self.attributes = []
        if attributes:
            self.attributes.extend(attributes)

    def expire_instance(self, instance):
        """Expire all memoized properties for *instance*."""
        stash = instance.__dict__
        for attribute in self.attributes:
            stash.pop(attribute, None)

    def __call__(self, fn):
        self.attributes.append(fn.__name__)
        return memoized_property(fn)

    def method(self, fn):
        self.attributes.append(fn.__name__)
        return memoized_instancemethod(fn)


class MemoizedSlots(object):
    """Apply memoized items to an object using a __getattr__ scheme.

    This allows the functionality of memoized_property and
    memoized_instancemethod to be available to a class using __slots__.

    """

    __slots__ = ()

    def _fallback_getattr(self, key):
        raise AttributeError(key)

    def __getattr__(self, key):
        if key.startswith('_memoized'):
            raise AttributeError(key)
        elif hasattr(self, '_memoized_attr_%s' % key):
            value = getattr(self, '_memoized_attr_%s' % key)()
            setattr(self, key, value)
            return value
        elif hasattr(self, '_memoized_method_%s' % key):
            fn = getattr(self, '_memoized_method_%s' % key)

            def oneshot(*args, **kw):
                result = fn(*args, **kw)
                memo = lambda *a, **kw: result
                memo.__name__ = fn.__name__
                memo.__doc__ = fn.__doc__
                setattr(self, key, memo)
                return result
            oneshot.__doc__ = fn.__doc__
            return oneshot
        else:
            return self._fallback_getattr(key)


def dependency_for(modulename):
    def decorate(obj):
        # TODO: would be nice to improve on this import silliness,
        # unfortunately importlib doesn't work that great either
        tokens = modulename.split(".")
        mod = compat.import_(
            ".".join(tokens[0:-1]), globals(), locals(), tokens[-1])
        mod = getattr(mod, tokens[-1])
        setattr(mod, obj.__name__, obj)
        return obj
    return decorate


class dependencies(object):
    """Apply imported dependencies as arguments to a function.

    E.g.::

        @util.dependencies(
            "sqlalchemy.sql.widget",
            "sqlalchemy.engine.default"
        );
        def some_func(self, widget, default, arg1, arg2, **kw):
            # ...

    Rationale is so that the impact of a dependency cycle can be
    associated directly with the few functions that cause the cycle,
    and not pollute the module-level namespace.

    """

    def __init__(self, *deps):
        self.import_deps = []
        for dep in deps:
            tokens = dep.split(".")
            self.import_deps.append(
                dependencies._importlater(
                    ".".join(tokens[0:-1]),
                    tokens[-1]
                )
            )

    def __call__(self, fn):
        import_deps = self.import_deps
        spec = compat.inspect_getfullargspec(fn)

        spec_zero = list(spec[0])
        hasself = spec_zero[0] in ('self', 'cls')

        for i in range(len(import_deps)):
            spec[0][i + (1 if hasself else 0)] = "import_deps[%r]" % i

        inner_spec = format_argspec_plus(spec, grouped=False)

        for impname in import_deps:
            del spec_zero[1 if hasself else 0]
        spec[0][:] = spec_zero

        outer_spec = format_argspec_plus(spec, grouped=False)

        code = 'lambda %(args)s: fn(%(apply_kw)s)' % {
            "args": outer_spec['args'],
            "apply_kw": inner_spec['apply_kw']
        }

        decorated = eval(code, locals())
        decorated.__defaults__ = getattr(fn, 'im_func', fn).__defaults__
        return update_wrapper(decorated, fn)

    @classmethod
    def resolve_all(cls, path):
        for m in list(dependencies._unresolved):
            if m._full_path.startswith(path):
                m._resolve()

    _unresolved = set()
    _by_key = {}

    class _importlater(object):
        _unresolved = set()

        _by_key = {}

        def __new__(cls, path, addtl):
            key = path + "." + addtl
            if key in dependencies._by_key:
                return dependencies._by_key[key]
            else:
                dependencies._by_key[key] = imp = object.__new__(cls)
                return imp

        def __init__(self, path, addtl):
            self._il_path = path
            self._il_addtl = addtl
            dependencies._unresolved.add(self)

        @property
        def _full_path(self):
            return self._il_path + "." + self._il_addtl

        @memoized_property
        def module(self):
            if self in dependencies._unresolved:
                raise ImportError(
                    "importlater.resolve_all() hasn't "
                    "been called (this is %s %s)"
                    % (self._il_path, self._il_addtl))

            return getattr(self._initial_import, self._il_addtl)

        def _resolve(self):
            dependencies._unresolved.discard(self)
            self._initial_import = compat.import_(
                self._il_path, globals(), locals(),
                [self._il_addtl])

        def __getattr__(self, key):
            if key == 'module':
                raise ImportError("Could not resolve module %s"
                                  % self._full_path)
            try:
                attr = getattr(self.module, key)
            except AttributeError:
                raise AttributeError(
                    "Module %s has no attribute '%s'" %
                    (self._full_path, key)
                )
            self.__dict__[key] = attr
            return attr


# from paste.deploy.converters
def asbool(obj):
    if isinstance(obj, compat.string_types):
        obj = obj.strip().lower()
        if obj in ['true', 'yes', 'on', 'y', 't', '1']:
            return True
        elif obj in ['false', 'no', 'off', 'n', 'f', '0']:
            return False
        else:
            raise ValueError("String is not true/false: %r" % obj)
    return bool(obj)


def bool_or_str(*text):
    """Return a callable that will evaluate a string as
    boolean, or one of a set of "alternate" string values.

    """
    def bool_or_value(obj):
        if obj in text:
            return obj
        else:
            return asbool(obj)
    return bool_or_value


def asint(value):
    """Coerce to integer."""

    if value is None:
        return value
    return int(value)


def coerce_kw_type(kw, key, type_, flexi_bool=True):
    r"""If 'key' is present in dict 'kw', coerce its value to type 'type\_' if
    necessary.  If 'flexi_bool' is True, the string '0' is considered false
    when coercing to boolean.
    """

    if key in kw and not isinstance(kw[key], type_) and kw[key] is not None:
        if type_ is bool and flexi_bool:
            kw[key] = asbool(kw[key])
        else:
            kw[key] = type_(kw[key])


def constructor_copy(obj, cls, *args, **kw):
    """Instantiate cls using the __dict__ of obj as constructor arguments.

    Uses inspect to match the named arguments of ``cls``.

    """

    names = get_cls_kwargs(cls)
    kw.update(
        (k, obj.__dict__[k]) for k in names.difference(kw)
        if k in obj.__dict__)
    return cls(*args, **kw)


def counter():
    """Return a threadsafe counter function."""

    lock = compat.threading.Lock()
    counter = itertools.count(1)

    # avoid the 2to3 "next" transformation...
    def _next():
        lock.acquire()
        try:
            return next(counter)
        finally:
            lock.release()

    return _next


def duck_type_collection(specimen, default=None):
    """Given an instance or class, guess if it is or is acting as one of
    the basic collection types: list, set and dict.  If the __emulates__
    property is present, return that preferentially.
    """

    if hasattr(specimen, '__emulates__'):
        # canonicalize set vs sets.Set to a standard: the builtin set
        if (specimen.__emulates__ is not None and
                issubclass(specimen.__emulates__, set)):
            return set
        else:
            return specimen.__emulates__

    isa = isinstance(specimen, type) and issubclass or isinstance
    if isa(specimen, list):
        return list
    elif isa(specimen, set):
        return set
    elif isa(specimen, dict):
        return dict

    if hasattr(specimen, 'append'):
        return list
    elif hasattr(specimen, 'add'):
        return set
    elif hasattr(specimen, 'set'):
        return dict
    else:
        return default


def assert_arg_type(arg, argtype, name):
    if isinstance(arg, argtype):
        return arg
    else:
        if isinstance(argtype, tuple):
            raise exc.ArgumentError(
                "Argument '%s' is expected to be one of type %s, got '%s'" %
                (name, ' or '.join("'%s'" % a for a in argtype), type(arg)))
        else:
            raise exc.ArgumentError(
                "Argument '%s' is expected to be of type '%s', got '%s'" %
                (name, argtype, type(arg)))


def dictlike_iteritems(dictlike):
    """Return a (key, value) iterator for almost any dict-like object."""

    if compat.py3k:
        if hasattr(dictlike, 'items'):
            return list(dictlike.items())
    else:
        if hasattr(dictlike, 'iteritems'):
            return dictlike.iteritems()
        elif hasattr(dictlike, 'items'):
            return iter(dictlike.items())

    getter = getattr(dictlike, '__getitem__', getattr(dictlike, 'get', None))
    if getter is None:
        raise TypeError(
            "Object '%r' is not dict-like" % dictlike)

    if hasattr(dictlike, 'iterkeys'):
        def iterator():
            for key in dictlike.iterkeys():
                yield key, getter(key)
        return iterator()
    elif hasattr(dictlike, 'keys'):
        return iter((key, getter(key)) for key in dictlike.keys())
    else:
        raise TypeError(
            "Object '%r' is not dict-like" % dictlike)


class classproperty(property):
    """A decorator that behaves like @property except that operates
    on classes rather than instances.

    The decorator is currently special when using the declarative
    module, but note that the
    :class:`~.sqlalchemy.ext.declarative.declared_attr`
    decorator should be used for this purpose with declarative.

    """

    def __init__(self, fget, *arg, **kw):
        super(classproperty, self).__init__(fget, *arg, **kw)
        self.__doc__ = fget.__doc__

    def __get__(desc, self, cls):
        return desc.fget(cls)


class hybridproperty(object):
    def __init__(self, func):
        self.func = func

    def __get__(self, instance, owner):
        if instance is None:
            clsval = self.func(owner)
            clsval.__doc__ = self.func.__doc__
            return clsval
        else:
            return self.func(instance)


class hybridmethod(object):
    """Decorate a function as cls- or instance- level."""

    def __init__(self, func):
        self.func = func

    def __get__(self, instance, owner):
        if instance is None:
            return self.func.__get__(owner, owner.__class__)
        else:
            return self.func.__get__(instance, owner)


class _symbol(int):
    def __new__(self, name, doc=None, canonical=None):
        """Construct a new named symbol."""
        assert isinstance(name, compat.string_types)
        if canonical is None:
            canonical = hash(name)
        v = int.__new__(_symbol, canonical)
        v.name = name
        if doc:
            v.__doc__ = doc
        return v

    def __reduce__(self):
        return symbol, (self.name, "x", int(self))

    def __str__(self):
        return repr(self)

    def __repr__(self):
        return "symbol(%r)" % self.name

_symbol.__name__ = 'symbol'


class symbol(object):
    """A constant symbol.

    >>> symbol('foo') is symbol('foo')
    True
    >>> symbol('foo')
    <symbol 'foo>

    A slight refinement of the MAGICCOOKIE=object() pattern.  The primary
    advantage of symbol() is its repr().  They are also singletons.

    Repeated calls of symbol('name') will all return the same instance.

    The optional ``doc`` argument assigns to ``__doc__``.  This
    is strictly so that Sphinx autoattr picks up the docstring we want
    (it doesn't appear to pick up the in-module docstring if the datamember
    is in a different module - autoattribute also blows up completely).
    If Sphinx fixes/improves this then we would no longer need
    ``doc`` here.

    """
    symbols = {}
    _lock = compat.threading.Lock()

    def __new__(cls, name, doc=None, canonical=None):
        cls._lock.acquire()
        try:
            sym = cls.symbols.get(name)
            if sym is None:
                cls.symbols[name] = sym = _symbol(name, doc, canonical)
            return sym
        finally:
            symbol._lock.release()


_creation_order = 1


def set_creation_order(instance):
    """Assign a '_creation_order' sequence to the given instance.

    This allows multiple instances to be sorted in order of creation
    (typically within a single thread; the counter is not particularly
    threadsafe).

    """
    global _creation_order
    instance._creation_order = _creation_order
    _creation_order += 1


def warn_exception(func, *args, **kwargs):
    """executes the given function, catches all exceptions and converts to
    a warning.

    """
    try:
        return func(*args, **kwargs)
    except Exception:
        warn("%s('%s') ignored" % sys.exc_info()[0:2])


def ellipses_string(value, len_=25):
    try:
        if len(value) > len_:
            return "%s..." % value[0:len_]
        else:
            return value
    except TypeError:
        return value


class _hash_limit_string(compat.text_type):
    """A string subclass that can only be hashed on a maximum amount
    of unique values.

    This is used for warnings so that we can send out parameterized warnings
    without the __warningregistry__ of the module,  or the non-overridable
    "once" registry within warnings.py, overloading memory,


    """
    def __new__(cls, value, num, args):
        interpolated = (value % args) + \
            (" (this warning may be suppressed after %d occurrences)" % num)
        self = super(_hash_limit_string, cls).__new__(cls, interpolated)
        self._hash = hash("%s_%d" % (value, hash(interpolated) % num))
        return self

    def __hash__(self):
        return self._hash

    def __eq__(self, other):
        return hash(self) == hash(other)


def warn(msg):
    """Issue a warning.

    If msg is a string, :class:`.exc.SAWarning` is used as
    the category.

    """
    warnings.warn(msg, exc.SAWarning, stacklevel=2)


def warn_limited(msg, args):
    """Issue a warning with a paramterized string, limiting the number
    of registrations.

    """
    if args:
        msg = _hash_limit_string(msg, 10, args)
    warnings.warn(msg, exc.SAWarning, stacklevel=2)


def only_once(fn):
    """Decorate the given function to be a no-op after it is called exactly
    once."""

    once = [fn]

    def go(*arg, **kw):
        if once:
            once_fn = once.pop()
            return once_fn(*arg, **kw)

    return go


_SQLA_RE = re.compile(r'sqlalchemy/([a-z_]+/){0,2}[a-z_]+\.py')
_UNITTEST_RE = re.compile(r'unit(?:2|test2?/)')


def chop_traceback(tb, exclude_prefix=_UNITTEST_RE, exclude_suffix=_SQLA_RE):
    """Chop extraneous lines off beginning and end of a traceback.

    :param tb:
      a list of traceback lines as returned by ``traceback.format_stack()``

    :param exclude_prefix:
      a regular expression object matching lines to skip at beginning of
      ``tb``

    :param exclude_suffix:
      a regular expression object matching lines to skip at end of ``tb``
    """
    start = 0
    end = len(tb) - 1
    while start <= end and exclude_prefix.search(tb[start]):
        start += 1
    while start <= end and exclude_suffix.search(tb[end]):
        end -= 1
    return tb[start:end + 1]

NoneType = type(None)


def attrsetter(attrname):
    code = \
        "def set(obj, value):"\
        "    obj.%s = value" % attrname
    env = locals().copy()
    exec(code, env)
    return env['set']


class EnsureKWArgType(type):
    """Apply translation of functions to accept **kw arguments if they
    don't already.

    """
    def __init__(cls, clsname, bases, clsdict):
        fn_reg = cls.ensure_kwarg
        if fn_reg:
            for key in clsdict:
                m = re.match(fn_reg, key)
                if m:
                    fn = clsdict[key]
                    spec = compat.inspect_getargspec(fn)
                    if not spec.keywords:
                        clsdict[key] = wrapped = cls._wrap_w_kw(fn)
                        setattr(cls, key, wrapped)
        super(EnsureKWArgType, cls).__init__(clsname, bases, clsdict)

    def _wrap_w_kw(self, fn):

        def wrap(*arg, **kw):
            return fn(*arg)
        return update_wrapper(wrap, fn)


def wrap_callable(wrapper, fn):
    """Augment functools.update_wrapper() to work with objects with
    a ``__call__()`` method.

    :param fn:
      object with __call__ method

    """
    if hasattr(fn, '__name__'):
        return update_wrapper(wrapper, fn)
    else:
        _f = wrapper
        _f.__name__ = fn.__class__.__name__
        if hasattr(fn, '__module__'):
            _f.__module__ = fn.__module__

        if hasattr(fn.__call__, '__doc__') and fn.__call__.__doc__:
            _f.__doc__ = fn.__call__.__doc__
        elif fn.__doc__:
            _f.__doc__ = fn.__doc__

        return _f